Supply devices are used whenever it is necessary to have direct-current supply voltages (of substantially constant value, irrespective of a load connected). In some applications such as, for example, driver circuits for piezoelectric actuators, a dual supply device is required to provide a double supply and, in particular, a positive supply voltage and a negative supply voltage of opposite values relative to a reference value (or ground); the positive and negative supply voltages are typically obtained from a main supply voltage of a different absolute value.
A known solution consists of the use of two direct-current/direct current (dc/dc) converters or choppers, which obtain the positive supply voltage and the negative supply voltage, respectively, from the main supply voltage. The dc/dc converters are formed by switching systems. Each dc/dc converter comprises a control circuit which switches a power transistor so as to generate a voltage with a rectangular wave form and with a predetermined mean value; and filtering elements, typically comprising an inductor and a capacitor, that enable a direct-current voltage (equal to this mean value) to be obtained from the rectangular wave-form voltage.
A disadvantage of the supply device described above is that it requires two separate control circuits, which are quite complex. Moreover, the use of two inductors makes the supply device extremely expensive, particularly when all of its other components are produced in integrated form in a single chip of semiconductor material.
A different known solution consists of the use of a single inductor with a central tap (or an autotransformer). It is thus possible to have a single dc/dc converter (with a single control circuit), the power transistor of which is connected to a terminal of the inductor (with the central tap connected to ground). The positive supply voltage and the negative supply voltage are thus obtained from the main supply voltage at the two terminals of the inductor, respectively.
However, the need to use an inductor with a central tap or an autotransformer (instead of a normal inductor) keeps the cost of this supply device quite high.